In general, MIG arc welding heretofore has used a mixed gas containing an appropriate amount (usually several %) of active gas, such as oxygen or carbon dioxide gas in an inert gas, such as argon gas in order to stabilize the arc, with positive electrode system, that is direct current reverse polarity wherein a welding torch is connected to a positive electrode of a direct current power supply and a mother metal to be welded is connected to a negative electrode.
The reason why the above described active gas component should be mixed in the shield gas is as follows. The first reason is that in the pure inert gas atmosphere added with no active gas component, arc roots (cathode spots) do not take fixed positions on mother plates to be welded and moves around on the mother plate surface sputtering from mill scales present on the mother metal surface, so that the arc trembles, the welding wire is irregularly fused and therefore the satisfactory welding result cannot be obtained. The second reason is that in the welded metal obtained with the arc as described above, the finger-shape weld penetration is formed at the bottom of a weld puddle and at this portion, the insufficient fusion, slag inclusion, blow holes and other welding defects are caused and the mixing of the active gas component has been only one means for preventing these defects.
In view of insurance of safety of welded joints, the above described defects cannot be accepted, so that as the means for obtaining the sound welded joints having no such defects, it has been essential to add an appropriate amount of active gas to an inert gas to stabilize the arc and prevent the defects.
The mixing ratio of the active gas is not always constant depending upon the object of welding, the kind of welding wire and the mother metal to be welded but, for example, it has been well known that Sigma grade argon (Linde Co. Ltd. U.S.A.) containing 1.about.5% of oxygen in argon gas is well-known and is effective to transfer the molten droplets at the wire tip in spray form.
Thus, the mixing of the above described active gas to the inert gas is effective for stabilizing the welding arc and preventing the welding defects. However, the oxygen content in the welded metal is inevitably increased and, for example, in the welding of 9% Ni steel requiring a high toughness at an ultra low temperature of about -196.degree. C. by using a wire having similar composition of a mother metal (ferritic wire), this increase of oxygen amount cannot satisfy the requirement of the toughness and the mixing of the active gas is not accepted in the welding wherein the toughness is particularly important.
As mentioned above, the prior MIG arc welding has practically used only with the direct current reverse polarity, because the molten droplets of the wire is smoothly transferred to the mother metal to be welded only with the reverse polarity and the good welded bead is formed, while, with the direct current straight polarity wherein the welding torch is connected to a negative electrode of the direct current power supply and the mother metal to be welded is connected to a positive electrode, a strong arc force acts against the molten droplets of the wire to push up the molten droplets, so that the molten droplets are transferred to the mother metal in such a form that the molten droplets are torn off, and result in large spatters and the formation of practically usable beads is substantially impossible.
In the alternating current MIG arc welding wherein the polarity is periodically alternated, the molten droplets are unstably transferred in the half wave wherein the polarity becomes the straight polarity and hence the formation of the good head is hampered, and further when the current value becomes zero at the alternating points, where the arc is once suppressed, a high voltage of about 300 V is necessary as a reignition voltage to reignite the arc and a large capacity of welding power supply is needed and therefore the danger when the welding operator is attacked by the electric shock is high, so that the above described high voltage power supply is not practically accepted.
From the above described reasons, MIG arc welding in the direct current straight polarity or alternating current has never been practically taken into consideration.
The present invention aims to provide MIG arc welding method in pure inert gas atmosphere in which the welding arc is stabilized and the molten droplets of the wire are smoothly transferred to the molten pool on the mother metal, in the direct current reverse polarity, the direct current straight polarity and the alternating current.